CHAPTER FOUR Meteorite Craters Nobody has ever witnessed the formation of a meteor­ meteorites, and (iii) rapidly solidified metallic droplets, ite crater. Interpretations must therefore be based upon analogous to the spheroids encountered in the vicinity of measurement and comparison with artificial craters, caused Meteor Crater, Arizona (Goldstein et al. 1972; Anders et al. by known magnitudes and depths of explosives. Excellent 1973); see page 397. studies have been performed by Baldwin (1949; 1963; The bulk of the typical large lunar craters were formed 1970) who was particularly interested in the puzzling when the kinetic energy ~mv 2 of the impacting body was problems associated with the lunar craters but, as a basis for converted into thermal energy within a fraction of a his speculations, thoroughly examined several terrestrial second, resulting in an explosion. There is a very high craters and presented extensive bibliographies. Results from probability that the impacting body was thereby itself nuclear test sites have been presented by Hansen (1968) totally destroyed, melted or vaporized (Hartman & and Short (1968a, b). Krinov (1960b; 1966a, b) has dis­ Wood 1971; Ahrens & O'Keefe 1972). Fortunately, for the cussed several craters and impact holes associated with science of meteoritics and for the inhabitants of Earth, our meteorites and also devoted a liuge chapter to the Tunguska atmosphere will alleviate the impact of celestial bodies and comet, which did not produce craters at all. See page 9. through a gradual deceleration cause an important propor­ Stanyukovich &' Fedynski (1947), Nininger (1952a; 1956), tion to survive as meteorites. However, large and dense Shoemaker (1963) and Gault et al. (1968) have made many bodies, page 22, will penetrate the atmosphere and impact pioneering studies of cratering and discussed the associated the Earth with a significant fraction of their initial energy problems from widely varying standpoints. Critical lists of still intact. Opik (I 951; 1958a) has examined the statistical terrestrial meteorite craters and recent bibliographies may probability that Earth is impacted by asteroidal and be found in Hey (1966: 538), Short & Bunch (1968) and cometary bodies, Table 16, and has also calculated the U.S. Geological Survey, Bulletin No. 1320 (1969). lethal effect of the collisions on land life. Opik even On the Moon and planetary bodies without an atmo­ speculated that the development of land life during the sphere, even very small particles will, upon impact, create Pre-Cambrian period may have been handicapped by craters. The surfaces of lunar material from the first catastrophic collisions as well as by other causes. Apollo 11 mission in 1969 could thus be shown to be ~puttered with small pits or craters, probably caused by For the sake of clarity it should be noted here that micrometeorites. (Neukum et al. 1970). Examination of giant meteorites can form two types of craters. The smaller Table 16. Collision Frequencies and Destructive effects Diameter of Impacting Body, km. 0.13 0.52 4.2 34 1.8·10 5 9.8·10-7 1.2 ·10-8 1.6·10-10 Collisions per ~ Comets Mars Asteroids 4.0·10-7 4.5·10-8 1.6·10-9 6.1·10-u year with Apollo Asteroids 2.8·10-5 6.6·10-7 2.4·10-9 8.0·10-u Total collisions per year 4.6·10-5 1.7·10-6 1.6·10-8 2.3·10-10 Interval between collisions, years 2.2·104 5.9·105 6.1·107 4.4-109 Lethal area, km2, at v ~ = 20km/sec 20 1300 5.6·105 9.6·107 lunar surface material recovered by the Apollo missions - a crater is more properly called a large impact hole and is total of 380 kg was secured by Apollo 11, 12, 14, 15, 16 generated by relatively small meteorites (<50 ton) with and 17 - has already revealed much meteoritic debris, relatively low velocities not exceeding 5 km/sec. Such processed by repeated cratering, "gardening." According to meteorites cause mechanical destruction of the ground and structural and chemical studies, the recognizable debris are themselves usually broken into a number of fragments consists mainly of (i) meteoritic material which was upon impact. The major part of the meteoritic fragments metamorphosed while incorporated in the lunar rocks, will remain in the impact hole mixed with shattered rock (ii) shock-altered fragments of chondrites and iron and soil. Typical examples are the 100-1,700 kg iron 34 Meteorite Craters meteorites of the Sikhote-Alin shower that produced impact holes 6-27 m in diameter and buried themselves to Diameter depths of 2-8 m (page 1123). 10 0 km The genuine craters discussed here are more than 100 m in diameter and were formed as the result of an explosion at the moment of impact. The projectile itself 10 km vaporized almost entirely, and tremendous shock waves raced outward from the focus. The largest explosions 1km produced ring anticlines and synclines surrounding an upraised central dome. On Earth these features have been severely modified by subsequent erosion or glaciation but 100m are observed to perfection on the Moon. At the time of impact the rock was crushed to a rock flour (see Canyon 10m Diablo), and the quartz minerals formed coesite (Chao eta!. 1960; Stoffler & Arndt 1969), stishovite (Chao eta!. 1962; Ida eta!. 1967) or lechatelierite (Rogers 1930). 1m Glasses formed from the desert sand (Wabar, Henbury) and fallout breccias, called suevites, formed as extensive blan­ 10cm kets over the crater basin (Ries Kessel ; Engelhardt & Stoffler 1968). Shatter cones (Dietz 1963; 1968) are now known from 1cm 10 em 1m 10m 100m 1km 10km 20 of the 50 recognized meteorite craters. They are striated Depth cup-and-cone structures which are best developed in dolom­ Figure 21. To a first approximation the dimensions of craters vary ite and other carbonate rocks but also occur in shale, in proportion to the cube root of the expended energy. The log-Jog sandstone, quartzite and granite. The cones range in size plot shown here indicates for a variety of explosion craters and meteoritic craters on Earth and Moon the diameter versus apparent from 1 em to 2 m and generally seem to have their apex depth (i.e., distance from rim crest to exposed bottom). (Adapted pointed towards the ground zero of the explosion. They from Baldwin 1963.) evidently formed by the passage of intense shock waves associated with the cratering impact. diameter, before it was brought to a full stop, he calculated Opik (1958a) estimated the effect of impact upon the the required energies and velocities to produce craters of a terrestrial rocks and found the following equations to be given size. See Figure 21 and Table 17. valid for impacting velocities, v, above 15 km/sec: The first crater on Earth which was widely - although Volume of vaporized rock: Vc == 0.284 v d 3 km 3 not universally - accepted as being of meteoritic origin was Volume of melted rock: VM == 0.224 v d 3 km 3 "Meteor Crater" or "Barringer Crater," around which the Volume of crushed rock: Vc==7.3vd3 km 3 , Canyon Diablo meteorites were found. From its history which is summarized on page 381. it is quite clear that a where d is the diameter in km of the impacting (stone) wealth of scientific ideas have been tested here since meteorite. Barringer (1905) first proposed its meteoritic nature. By a Baldwin (1949; 1963; 1971) analyzed the crate ring curious coincidence the next half a dozen craters were all situation somewhat differently, extending his own results recognized in the ten years between 1928-1937, since from TNT explosives to meteorite impacts on the Earth and which time only one crater, Wolf Creek, associated with Moon. On the assumption that the impacting meteorite meteoritic debris has been reported.* However, other, more only penetrated to shallow depths, equal to twice its own ancient craters without meteorites have been discovered since the 1950s due to a concerted effort particularly by Table 17. Sizes of iron meteorites which could produce certain Canadian scientists (see, e.g., Hartmann 1965; De nee craters if they exploded in "average soil" at a scaled depth of burst: eta!. 1968; Freeberg 1969). At least 16 craters on the Hem= 0.04 VE cal. Adapted from Baldwin 1963:165, 175, 447. Canadian shield are now well documented, many of them Crater Log Energy Velocity at impact, vi Time to being of Pre-Cambrian age. The Charlevoix (La Malbaie) Diameter E calories 3.2 km/sec 16 km/sec Slop. meter (I kg TNT - Log mass Diameter Log ma ss Diameter seconds crater, on the north shore of the St. Lawrence River is 106 cal.) ton meter ton meter v; ; 16 km /sec 35-40 km in diameter, but it is bisected by the river and 9.5 8.88 -0.2 1 0. 51 - 1.6 1 0 .1 8 0.000045 deeply eroded, so that it was not recognized until the 95 12. 13 3.34 6.3 1.64 2. 16 0.000545 950 15.55 6.46 87 5.06 29.7 0.0075 0 presence of shatter cones was noted (Rondot 1968; 9500 19. 18 10.09 14 10 8.69 483 0. 122 Robertson 1968). The Manicouagan crater in Quebec, 95000 23 06 13.97 27750 12.57 9480 2.39 which is of Palaeozoic age and now a lake, is , with its Baldwin's equations and curves will in general indicate that a given crater may be produced by meteorites an order of magnitude *In the Supplement the author has added one more such crater, smaller than those derived from Opik's equations.